Amiga Magazin: Amiga-CD 1996 July/AMIGA_1996_7.BIN/ausgabe_7_96/pd-programmierung/perl5_002bin.lha/man/catp/perlre.0

Text File | 1996-03-02 | 46KB | 727 lines

PERLRE(1)      User Contributed Perl Documentation      PERLRE(1)


NNNNAAAAMMMMEEEE
       perlre - Perl regular expressions

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
       This page describes the syntax of regular expressions in
       Perl.  For a description of how to actually _u_s_e regular
       expressions in matching operations, plus various examples
       of the same, see mmmm//////// and ssss//////////// in the _p_e_r_l_o_p manpage.

       The matching operations can have various modifiers, some
       of which relate to the interpretation of the regular
       expression inside.  These are:

           iiii   DDDDoooo ccccaaaasssseeee----iiiinnnnsssseeeennnnssssiiiittttiiiivvvveeee ppppaaaatttttttteeeerrrrnnnn mmmmaaaattttcccchhhhiiiinnnngggg....
           mmmm   TTTTrrrreeeeaaaatttt ssssttttrrrriiiinnnngggg aaaassss mmmmuuuullllttttiiiipppplllleeee lllliiiinnnneeeessss....
           ssss   TTTTrrrreeeeaaaatttt ssssttttrrrriiiinnnngggg aaaassss ssssiiiinnnngggglllleeee lllliiiinnnneeee....
           xxxx   EEEExxxxtttteeeennnndddd yyyyoooouuuurrrr ppppaaaatttttttteeeerrrrnnnn''''ssss lllleeeeggggiiiibbbbiiiilllliiiittttyyyy wwwwiiiitttthhhh wwwwhhhhiiiitttteeeessssppppaaaacccceeee aaaannnndddd ccccoooommmmmmmmeeeennnnttttssss....

       These are usually written as "the ////xxxx modifier", even
       though the delimiter in question might not actually be a
       slash.  In fact, any of these modifiers may also be
       embedded within the regular expression itself using the
       new ((((????............)))) construct.  See below.

       The ////xxxx modifier itself needs a little more explanation.
       It tells the regular expression parser to ignore
       whitespace that is not backslashed or within a character
       class.  You can use this to break up your regular
       expression into (slightly) more readable parts.  The ####
       character is also treated as a metacharacter introducing a
       comment, just as in ordinary Perl code.  Taken together,
       these features go a long way towards making Perl 5 a
       readable language.  See the C comment deletion code in the
       _p_e_r_l_o_p manpage.

       RRRReeeegggguuuullllaaaarrrr EEEExxxxpppprrrreeeessssssssiiiioooonnnnssss

       The patterns used in pattern matching are regular
       expressions such as those supplied in the Version 8 regexp
       routines.  (In fact, the routines are derived (distantly)
       from Henry Spencer's freely redistributable
       reimplementation of the V8 routines.)  See the section on
       _V_e_r_s_i_o_n _8 _R_e_g_u_l_a_r _E_x_p_r_e_s_s_i_o_n_s for details.

       In particular the following metacharacters have their
       standard _e_g_r_e_p-ish meanings:

           \\\\   QQQQuuuuooootttteeee tttthhhheeee nnnneeeexxxxtttt mmmmeeeettttaaaacccchhhhaaaarrrraaaacccctttteeeerrrr
           ^^^^   MMMMaaaattttcccchhhh tttthhhheeee bbbbeeeeggggiiiinnnnnnnniiiinnnngggg ooooffff tttthhhheeee lllliiiinnnneeee
           ....   MMMMaaaattttcccchhhh aaaannnnyyyy cccchhhhaaaarrrraaaacccctttteeeerrrr ((((eeeexxxxcccceeeepppptttt nnnneeeewwwwlllliiiinnnneeee))))
           $$$$   MMMMaaaattttcccchhhh tttthhhheeee eeeennnndddd ooooffff tttthhhheeee lllliiiinnnneeee ((((oooorrrr bbbbeeeeffffoooorrrreeee nnnneeeewwwwlllliiiinnnneeee aaaatttt tttthhhheeee eeeennnndddd))))
           ||||   AAAAlllltttteeeerrrrnnnnaaaattttiiiioooonnnn
           (((())))  GGGGrrrroooouuuuppppiiiinnnngggg
           [[[[]]]]  CCCChhhhaaaarrrraaaacccctttteeeerrrr ccccllllaaaassssssss



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PERLRE(1)      User Contributed Perl Documentation      PERLRE(1)


       By default, the "^" character is guaranteed to match only
       at the beginning of the string, the "$" character only at
       the end (or before the newline at the end) and Perl does
       certain optimizations with the assumption that the string
       contains only one line.  Embedded newlines will not be
       matched by "^" or "$".  You may, however, wish to treat a
       string as a multi-line buffer, such that the "^" will
       match after any newline within the string, and "$" will
       match before any newline.  At the cost of a little more
       overhead, you can do this by using the /m modifier on the
       pattern match operator.  (Older programs did this by
       setting $$$$****, but this practice is deprecated in Perl 5.)

       To facilitate multi-line substitutions, the "." character
       never matches a newline unless you use the ////ssss modifier,
       which tells Perl to pretend the string is a single
       line--even if it isn't.  The ////ssss modifier also overrides
       the setting of $$$$****, in case you have some (badly behaved)
       older code that sets it in another module.

       The following standard quantifiers are recognized:

           ****      MMMMaaaattttcccchhhh 0000 oooorrrr mmmmoooorrrreeee ttttiiiimmmmeeeessss
           ++++      MMMMaaaattttcccchhhh 1111 oooorrrr mmmmoooorrrreeee ttttiiiimmmmeeeessss
           ????      MMMMaaaattttcccchhhh 1111 oooorrrr 0000 ttttiiiimmmmeeeessss
           {{{{nnnn}}}}    MMMMaaaattttcccchhhh eeeexxxxaaaaccccttttllllyyyy nnnn ttttiiiimmmmeeeessss
           {{{{nnnn,,,,}}}}   MMMMaaaattttcccchhhh aaaatttt lllleeeeaaaasssstttt nnnn ttttiiiimmmmeeeessss
           {{{{nnnn,,,,mmmm}}}}  MMMMaaaattttcccchhhh aaaatttt lllleeeeaaaasssstttt nnnn bbbbuuuutttt nnnnooootttt mmmmoooorrrreeee tttthhhhaaaannnn mmmm ttttiiiimmmmeeeessss

       (If a curly bracket occurs in any other context, it is
       treated as a regular character.)  The "*" modifier is
       equivalent to {{{{0000,,,,}}}}, the "+" modifier to {{{{1111,,,,}}}}, and the "?"
       modifier to {{{{0000,,,,1111}}}}.  n and m are limited to integral values
       less than 65536.

       By default, a quantified subpattern is "greedy", that is,
       it will match as many times as possible without causing
       the rest pattern not to match.  The standard quantifiers
       are all "greedy", in that they match as many occurrences
       as possible (given a particular starting location) without
       causing the pattern to fail.  If you want it to match the
       minimum number of times possible, follow the quantifier
       with a "?" after any of them.  Note that the meanings
       don't change, just the "gravity":

           ****????     MMMMaaaattttcccchhhh 0000 oooorrrr mmmmoooorrrreeee ttttiiiimmmmeeeessss
           ++++????     MMMMaaaattttcccchhhh 1111 oooorrrr mmmmoooorrrreeee ttttiiiimmmmeeeessss
           ????????     MMMMaaaattttcccchhhh 0000 oooorrrr 1111 ttttiiiimmmmeeee
           {{{{nnnn}}}}????   MMMMaaaattttcccchhhh eeeexxxxaaaaccccttttllllyyyy nnnn ttttiiiimmmmeeeessss
           {{{{nnnn,,,,}}}}????  MMMMaaaattttcccchhhh aaaatttt lllleeeeaaaasssstttt nnnn ttttiiiimmmmeeeessss
           {{{{nnnn,,,,mmmm}}}}???? MMMMaaaattttcccchhhh aaaatttt lllleeeeaaaasssstttt nnnn bbbbuuuutttt nnnnooootttt mmmmoooorrrreeee tttthhhhaaaannnn mmmm ttttiiiimmmmeeeessss

       Since patterns are processed as double quoted strings, the
       following also work:



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PERLRE(1)      User Contributed Perl Documentation      PERLRE(1)


           \\\\tttt          ttttaaaabbbb
           \\\\nnnn          nnnneeeewwwwlllliiiinnnneeee
           \\\\rrrr          rrrreeeettttuuuurrrrnnnn
           \\\\ffff          ffffoooorrrrmmmm ffffeeeeeeeedddd
           \\\\aaaa          aaaallllaaaarrrrmmmm ((((bbbbeeeellllllll))))
           \\\\eeee          eeeessssccccaaaappppeeee ((((tttthhhhiiiinnnnkkkk ttttrrrrooooffffffff))))
           \\\\000033333333        ooooccccttttaaaallll cccchhhhaaaarrrr ((((tttthhhhiiiinnnnkkkk ooooffff aaaa PPPPDDDDPPPP----11111111))))
           \\\\xxxx1111BBBB        hhhheeeexxxx cccchhhhaaaarrrr
           \\\\cccc[[[[         ccccoooonnnnttttrrrroooollll cccchhhhaaaarrrr
           \\\\llll          lllloooowwwweeeerrrrccccaaaasssseeee nnnneeeexxxxtttt cccchhhhaaaarrrr ((((tttthhhhiiiinnnnkkkk vvvviiii))))
           \\\\uuuu          uuuuppppppppeeeerrrrccccaaaasssseeee nnnneeeexxxxtttt cccchhhhaaaarrrr ((((tttthhhhiiiinnnnkkkk vvvviiii))))
           \\\\LLLL          lllloooowwwweeeerrrrccccaaaasssseeee ttttiiiillllllll \\\\EEEE ((((tttthhhhiiiinnnnkkkk vvvviiii))))
           \\\\UUUU          uuuuppppppppeeeerrrrccccaaaasssseeee ttttiiiillllllll \\\\EEEE ((((tttthhhhiiiinnnnkkkk vvvviiii))))
           \\\\EEEE          eeeennnndddd ccccaaaasssseeee mmmmooooddddiiiiffffiiiiccccaaaattttiiiioooonnnn ((((tttthhhhiiiinnnnkkkk vvvviiii))))
           \\\\QQQQ          qqqquuuuooootttteeee rrrreeeeggggeeeexxxxpppp mmmmeeeettttaaaacccchhhhaaaarrrraaaacccctttteeeerrrrssss ttttiiiillllllll \\\\EEEE

       In addition, Perl defines the following:

           \\\\wwww  MMMMaaaattttcccchhhh aaaa """"wwwwoooorrrrdddd"""" cccchhhhaaaarrrraaaacccctttteeeerrrr ((((aaaallllpppphhhhaaaannnnuuuummmmeeeerrrriiiicccc pppplllluuuussss """"____""""))))
           \\\\WWWW  MMMMaaaattttcccchhhh aaaa nnnnoooonnnn----wwwwoooorrrrdddd cccchhhhaaaarrrraaaacccctttteeeerrrr
           \\\\ssss  MMMMaaaattttcccchhhh aaaa wwwwhhhhiiiitttteeeessssppppaaaacccceeee cccchhhhaaaarrrraaaacccctttteeeerrrr
           \\\\SSSS  MMMMaaaattttcccchhhh aaaa nnnnoooonnnn----wwwwhhhhiiiitttteeeessssppppaaaacccceeee cccchhhhaaaarrrraaaacccctttteeeerrrr
           \\\\dddd  MMMMaaaattttcccchhhh aaaa ddddiiiiggggiiiitttt cccchhhhaaaarrrraaaacccctttteeeerrrr
           \\\\DDDD  MMMMaaaattttcccchhhh aaaa nnnnoooonnnn----ddddiiiiggggiiiitttt cccchhhhaaaarrrraaaacccctttteeeerrrr

       Note that \\\\wwww matches a single alphanumeric character, not
       a whole word.  To match a word you'd need to say \\\\wwww++++.  You
       may use \\\\wwww, \\\\WWWW, \\\\ssss, \\\\SSSS, \\\\dddd and \\\\DDDD within character classes
       (though not as either end of a range).

       Perl defines the following zero-width assertions:

           \\\\bbbb  MMMMaaaattttcccchhhh aaaa wwwwoooorrrrdddd bbbboooouuuunnnnddddaaaarrrryyyy
           \\\\BBBB  MMMMaaaattttcccchhhh aaaa nnnnoooonnnn----((((wwwwoooorrrrdddd bbbboooouuuunnnnddddaaaarrrryyyy))))
           \\\\AAAA  MMMMaaaattttcccchhhh oooonnnnllllyyyy aaaatttt bbbbeeeeggggiiiinnnnnnnniiiinnnngggg ooooffff ssssttttrrrriiiinnnngggg
           \\\\ZZZZ  MMMMaaaattttcccchhhh oooonnnnllllyyyy aaaatttt eeeennnndddd ooooffff ssssttttrrrriiiinnnngggg ((((oooorrrr bbbbeeeeffffoooorrrreeee nnnneeeewwwwlllliiiinnnneeee aaaatttt tttthhhheeee eeeennnndddd))))
           \\\\GGGG  MMMMaaaattttcccchhhh oooonnnnllllyyyy wwwwhhhheeeerrrreeee pppprrrreeeevvvviiiioooouuuussss mmmm////////gggg lllleeeefffftttt ooooffffffff

       A word boundary (\\\\bbbb) is defined as a spot between two
       characters that has a \\\\wwww on one side of it and and a \\\\WWWW on
       the other side of it (in either order), counting the
       imaginary characters off the beginning and end of the
       string as matching a \\\\WWWW.  (Within character classes \\\\bbbb
       represents backspace rather than a word boundary.)  The \\\\AAAA
       and \\\\ZZZZ are just like "^" and "$" except that they won't
       match multiple times when the ////mmmm modifier is used, while
       "^" and "$" will match at every internal line boundary.
       To match the actual end of the string, not ignoring
       newline, you can use \\\\ZZZZ((((????!!!!\\\\nnnn)))).

       When the bracketing construct (((( ............ )))) is used, \<digit>
       matches the digit'th substring.  Outside of the pattern,
       always use "$" instead of "\" in front of the digit.  (The
       \<digit> notation can on rare occasion work outside the



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PERLRE(1)      User Contributed Perl Documentation      PERLRE(1)


       current pattern, this should not be relied upon.  See the
       WARNING below.) The scope of $<digit> (and $$$$````, $$$$&&&&, and $$$$''''))))
       extends to the end of the enclosing BLOCK or eval string,
       or to the next successful pattern match, whichever comes
       first.  If you want to use parentheses to delimit
       subpattern (e.g. a set of alternatives) without saving it
       as a subpattern, follow the ( with a ?.

       You may have as many parentheses as you wish.  If you have
       more than 9 substrings, the variables $$$$11110000, $$$$11111111, ... refer
       to the corresponding substring.  Within the pattern, \10,
       \11, etc. refer back to substrings if there have been at
       least that many left parens before the backreference.
       Otherwise (for backward compatibility) \10 is the same as
       \010, a backspace, and \11 the same as \011, a tab.  And
       so on.  (\1 through \9 are always backreferences.)

       $$$$++++ returns whatever the last bracket match matched.  $$$$&&&&
       returns the entire matched string.  ($0 used to return the
       same thing, but not any more.)  $$$$```` returns everything
       before the matched string.  $$$$'''' returns everything after
       the matched string.  Examples:

           ssss////^^^^(((([[[[^^^^ ]]]]****)))) ****(((([[[[^^^^ ]]]]****))))////$$$$2222 $$$$1111////;;;;     #### sssswwwwaaaapppp ffffiiiirrrrsssstttt ttttwwwwoooo wwwwoooorrrrddddssss

           iiiiffff ((((////TTTTiiiimmmmeeee:::: ((((........))))::::((((........))))::::((((........))))////)))) {{{{
               $$$$hhhhoooouuuurrrrssss ==== $$$$1111;;;;
               $$$$mmmmiiiinnnnuuuutttteeeessss ==== $$$$2222;;;;
               $$$$sssseeeeccccoooonnnnddddssss ==== $$$$3333;;;;
           }}}}

       You will note that all backslashed metacharacters in Perl
       are alphanumeric, such as \\\\bbbb, \\\\wwww, \\\\nnnn.  Unlike some other
       regular expression languages, there are no backslashed
       symbols that aren't alphanumeric.  So anything that looks
       like \\, \(, \), \<, \>, \{, or \} is always interpreted
       as a literal character, not a metacharacter.  This makes
       it simple to quote a string that you want to use for a
       pattern but that you are afraid might contain
       metacharacters.  Simply quote all the non-alphanumeric
       characters:

           $$$$ppppaaaatttttttteeeerrrrnnnn ====~~~~ ssss////((((\\\\WWWW))))////\\\\\\\\$$$$1111////gggg;;;;

       You can also use the built-in _q_u_o_t_e_m_e_t_a_(_) function to do
       this.  An even easier way to quote metacharacters right in
       the match operator is to say

           ////$$$$uuuunnnnqqqquuuuooootttteeeedddd\\\\QQQQ$$$$qqqquuuuooootttteeeedddd\\\\EEEE$$$$uuuunnnnqqqquuuuooootttteeeedddd////

       Perl 5 defines a consistent extension syntax for regular
       expressions.  The syntax is a pair of parens with a
       question mark as the first thing within the parens (this
       was a syntax error in Perl 4).  The character after the



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PERLRE(1)      User Contributed Perl Documentation      PERLRE(1)


       question mark gives the function of the extension.
       Several extensions are already supported:

       (?#text)  A comment.  The text is ignored.  If the ////xxxx
                 switch is used to enable whitespace formatting,
                 a simple #### will suffice.

       (?:regexp)
                 This groups things like "()" but doesn't make
                 backrefences like "()" does.  So

                     sssspppplllliiiitttt((((////\\\\bbbb((((????::::aaaa||||bbbb||||cccc))))\\\\bbbb////))))

                 is like

                     sssspppplllliiiitttt((((////\\\\bbbb((((aaaa||||bbbb||||cccc))))\\\\bbbb////))))

                 but doesn't spit out extra fields.

       (?=regexp)
                 A zero-width positive lookahead assertion.  For
                 example, ////\\\\wwww++++((((????====\\\\tttt))))//// matches a word followed by
                 a tab, without including the tab in $$$$&&&&.

       (?!regexp)
                 A zero-width negative lookahead assertion.  For
                 example ////ffffoooooooo((((????!!!!bbbbaaaarrrr))))//// matches any occurrence of
                 "foo" that isn't followed by "bar".  Note
                 however that lookahead and lookbehind are NOT
                 the same thing.  You cannot use this for
                 lookbehind: ////((((????!!!!ffffoooooooo))))bbbbaaaarrrr//// will not find an
                 occurrence of "bar" that is preceded by
                 something which is not "foo".  That's because
                 the ((((????!!!!ffffoooooooo)))) is just saying that the next thing
                 cannot be "foo"--and it's not, it's a "bar", so
                 "foobar" will match.  You would have to do
                 something like ////((((????ffffoooooooo))))............bbbbaaaarrrr//// for that.   We say
                 "like" because there's the case of your "bar"
                 not having three characters before it.  You
                 could cover that this way:
                 ////((((????::::((((????!!!!ffffoooooooo))))............||||^^^^........????))))bbbbaaaarrrr////.  Sometimes it's still
                 easier just to say:

                     iiiiffff ((((////ffffoooooooo//// &&&&&&&& $$$$```` ====~~~~ ////bbbbaaaarrrr$$$$////))))


       (?imsx)   One or more embedded pattern-match modifiers.
                 This is particularly useful for patterns that
                 are specified in a table somewhere, some of
                 which want to be case sensitive, and some of
                 which don't.  The case insensitive ones merely
                 need to include ((((????iiii)))) at the front of the
                 pattern.  For example:




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                     $$$$ppppaaaatttttttteeeerrrrnnnn ==== """"ffffoooooooobbbbaaaarrrr"""";;;;
                     iiiiffff (((( ////$$$$ppppaaaatttttttteeeerrrrnnnn////iiii ))))

                     #### mmmmoooorrrreeee fffflllleeeexxxxiiiibbbblllleeee::::

                     $$$$ppppaaaatttttttteeeerrrrnnnn ==== """"((((????iiii))))ffffoooooooobbbbaaaarrrr"""";;;;
                     iiiiffff (((( ////$$$$ppppaaaatttttttteeeerrrrnnnn//// ))))


       The specific choice of question mark for this and the new
       minimal matching construct was because 1) question mark is
       pretty rare in older regular expressions, and 2) whenever
       you see one, you should stop and "question" exactly what
       is going on.  That's psychology...

       BBBBaaaacccckkkkttttrrrraaaacccckkkkiiiinnnngggg

       A fundamental feature of regular expression matching
       involves the notion called _b_a_c_k_t_r_a_c_k_i_n_g.  which is used
       (when needed) by all regular expression quantifiers,
       namely ****, ****????, ++++, ++++????, {{{{nnnn,,,,mmmm}}}}, and {{{{nnnn,,,,mmmm}}}}????.

       For a regular expression to match, the _e_n_t_i_r_e regular
       expression must match, not just part of it.  So if the
       beginning of a pattern containing a quantifier succeeds in
       a way that causes later parts in the pattern to fail, the
       matching engine backs up and recalculates the beginning
       part--that's why it's called backtracking.

       Here is an example of backtracking:  Let's say you want to
       find the word following "foo" in the string "Food is on
       the foo table.":

           $$$$____ ==== """"FFFFoooooooodddd iiiissss oooonnnn tttthhhheeee ffffoooooooo ttttaaaabbbblllleeee...."""";;;;
           iiiiffff (((( ////\\\\bbbb((((ffffoooooooo))))\\\\ssss++++((((\\\\wwww++++))))////iiii )))) {{{{
               pppprrrriiiinnnntttt """"$$$$2222 ffffoooolllllllloooowwwwssss $$$$1111....\\\\nnnn"""";;;;
           }}}}

       When the match runs, the first part of the regular
       expression (\\\\bbbb((((ffffoooooooo))))) finds a possible match right at the
       beginning of the string, and loads up $$$$1111 with "Foo".
       However, as soon as the matching engine sees that there's
       no whitespace following the "Foo" that it had saved in $$$$1111,
       it realizes its mistake and starts over again one
       character after where it had had the tentative match.
       This time it goes all the way until the next occurrence of
       "foo". The complete regular expression matches this time,
       and you get the expected output of "table follows foo."

       Sometimes minimal matching can help a lot.  Imagine you'd
       like to match everything between "foo" and "bar".
       Initially, you write something like this:





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           $$$$____ ====  """"TTTThhhheeee ffffoooooooodddd iiiissss uuuunnnnddddeeeerrrr tttthhhheeee bbbbaaaarrrr iiiinnnn tttthhhheeee bbbbaaaarrrrnnnn...."""";;;;
           iiiiffff (((( ////ffffoooooooo((((....****))))bbbbaaaarrrr//// )))) {{{{
               pppprrrriiiinnnntttt """"ggggooootttt <<<<$$$$1111>>>>\\\\nnnn"""";;;;
           }}}}

       Which perhaps unexpectedly yields:

         ggggooootttt <<<<dddd iiiissss uuuunnnnddddeeeerrrr tttthhhheeee bbbbaaaarrrr iiiinnnn tttthhhheeee >>>>

       That's because ....**** was greedy, so you get everything
       between the _f_i_r_s_t "foo" and the _l_a_s_t "bar".  In this case,
       it's more effective to use minimal matching to make sure
       you get the text between a "foo" and the first "bar"
       thereafter.

           iiiiffff (((( ////ffffoooooooo((((....****????))))bbbbaaaarrrr//// )))) {{{{ pppprrrriiiinnnntttt """"ggggooootttt <<<<$$$$1111>>>>\\\\nnnn"""" }}}}
         ggggooootttt <<<<dddd iiiissss uuuunnnnddddeeeerrrr tttthhhheeee >>>>

       Here's another example: let's say you'd like to match a
       number at the end of a string, and you also want to keep
       the preceding part the match.  So you write this:

           $$$$____ ==== """"IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: 55553333111144447777"""";;;;
           iiiiffff (((( ////((((....****))))((((\\\\dddd****))))//// )))) {{{{                                #### WWWWrrrroooonnnngggg!!!!
               pppprrrriiiinnnntttt """"BBBBeeeeggggiiiinnnnnnnniiiinnnngggg iiiissss <<<<$$$$1111>>>>,,,, nnnnuuuummmmbbbbeeeerrrr iiiissss <<<<$$$$2222>>>>....\\\\nnnn"""";;;;
           }}}}

       That won't work at all, because ....**** was greedy and gobbled
       up the whole string. As \\\\dddd**** can match on an empty string
       the complete regular expression matched successfully.

           BBBBeeeeggggiiiinnnnnnnniiiinnnngggg iiiissss <<<<IIII hhhhaaaavvvveeee 2222:::: 55553333111144447777>>>>,,,, nnnnuuuummmmbbbbeeeerrrr iiiissss <<<<>>>>....

       Here are some variants, most of which don't work:

           $$$$____ ==== """"IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: 55553333111144447777"""";;;;
           @@@@ppppaaaattttssss ==== qqqqwwww{{{{
               ((((....****))))((((\\\\dddd****))))
               ((((....****))))((((\\\\dddd++++))))
               ((((....****????))))((((\\\\dddd****))))
               ((((....****????))))((((\\\\dddd++++))))
               ((((....****))))((((\\\\dddd++++))))$$$$
               ((((....****????))))((((\\\\dddd++++))))$$$$
               ((((....****))))\\\\bbbb((((\\\\dddd++++))))$$$$
               ((((....****\\\\DDDD))))((((\\\\dddd++++))))$$$$
           }}}};;;;











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           ffffoooorrrr $$$$ppppaaaatttt ((((@@@@ppppaaaattttssss)))) {{{{
               pppprrrriiiinnnnttttffff """"%%%%----11112222ssss """",,,, $$$$ppppaaaatttt;;;;
               iiiiffff (((( ////$$$$ppppaaaatttt//// )))) {{{{
                   pppprrrriiiinnnntttt """"<<<<$$$$1111>>>> <<<<$$$$2222>>>>\\\\nnnn"""";;;;
               }}}} eeeellllsssseeee {{{{
                   pppprrrriiiinnnntttt """"FFFFAAAAIIIILLLL\\\\nnnn"""";;;;
               }}}}
           }}}}

       That will print out:

           ((((....****))))((((\\\\dddd****))))    <<<<IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: 55553333111144447777>>>> <<<<>>>>
           ((((....****))))((((\\\\dddd++++))))    <<<<IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: 5555333311114444>>>> <<<<7777>>>>
           ((((....****????))))((((\\\\dddd****))))   <<<<>>>> <<<<>>>>
           ((((....****????))))((((\\\\dddd++++))))   <<<<IIII hhhhaaaavvvveeee >>>> <<<<2222>>>>
           ((((....****))))((((\\\\dddd++++))))$$$$   <<<<IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: 5555333311114444>>>> <<<<7777>>>>
           ((((....****????))))((((\\\\dddd++++))))$$$$  <<<<IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: >>>> <<<<55553333111144447777>>>>
           ((((....****))))\\\\bbbb((((\\\\dddd++++))))$$$$ <<<<IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: >>>> <<<<55553333111144447777>>>>
           ((((....****\\\\DDDD))))((((\\\\dddd++++))))$$$$ <<<<IIII hhhhaaaavvvveeee 2222 nnnnuuuummmmbbbbeeeerrrrssss:::: >>>> <<<<55553333111144447777>>>>

       As you see, this can be a bit tricky.  It's important to
       realize that a regular expression is merely a set of
       assertions that gives a definition of success.  There may
       be 0, 1, or several different ways that the definition
       might succeed against a particular string.  And if there
       are multiple ways it might succeed, you need to understand
       backtracking in order to know which variety of success you
       will achieve.

       When using lookahead assertions and negations, this can
       all get even tricker.  Imagine you'd like to find a
       sequence of nondigits not followed by "123".  You might
       try to write that as

               $$$$____ ==== """"AAAABBBBCCCC111122223333"""";;;;
               iiiiffff (((( ////^^^^\\\\DDDD****((((????!!!!111122223333))))//// )))) {{{{                          #### WWWWrrrroooonnnngggg!!!!
                   pppprrrriiiinnnntttt """"YYYYuuuupppp,,,, nnnnoooo 111122223333 iiiinnnn $$$$____\\\\nnnn"""";;;;
               }}}}

       But that isn't going to match; at least, not the way
       you're hoping.  It claims that there is no 123 in the
       string.  Here's a clearer picture of why it that pattern
       matches, contrary to popular expectations:

           $$$$xxxx ==== ''''AAAABBBBCCCC111122223333'''' ;;;;
           $$$$yyyy ==== ''''AAAABBBBCCCC444444445555'''' ;;;;

           pppprrrriiiinnnntttt """"1111:::: ggggooootttt $$$$1111\\\\nnnn"""" iiiiffff $$$$xxxx ====~~~~ ////^^^^((((AAAABBBBCCCC))))((((????!!!!111122223333))))//// ;;;;
           pppprrrriiiinnnntttt """"2222:::: ggggooootttt $$$$1111\\\\nnnn"""" iiiiffff $$$$yyyy ====~~~~ ////^^^^((((AAAABBBBCCCC))))((((????!!!!111122223333))))//// ;;;;

           pppprrrriiiinnnntttt """"3333:::: ggggooootttt $$$$1111\\\\nnnn"""" iiiiffff $$$$xxxx ====~~~~ ////^^^^((((\\\\DDDD****))))((((????!!!!111122223333))))//// ;;;;
           pppprrrriiiinnnntttt """"4444:::: ggggooootttt $$$$1111\\\\nnnn"""" iiiiffff $$$$yyyy ====~~~~ ////^^^^((((\\\\DDDD****))))((((????!!!!111122223333))))//// ;;;;

       This prints



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           2222:::: ggggooootttt AAAABBBBCCCC
           3333:::: ggggooootttt AAAABBBB
           4444:::: ggggooootttt AAAABBBBCCCC

       You might have expected test 3 to fail because it just
       seems to a more general purpose version of test 1.  The
       important difference between them is that test 3 contains
       a quantifier (\\\\DDDD****) and so can use backtracking, whereas
       test 1 will not.  What's happening is that you've asked
       "Is it true that at the start of $$$$xxxx, following 0 or more
       nondigits, you have something that's not 123?"  If the
       pattern matcher had let \\\\DDDD**** expand to "ABC", this would
       have caused the whole pattern to fail.  The search engine
       will initially match \\\\DDDD**** with "ABC".  Then it will try to
       match ((((????!!!!111122223333 with "123" which, of course, fails.  But
       because a quantifier (\\\\DDDD****) has been used in the regular
       expression, the search engine can backtrack and retry the
       match differently in the hope of matching the complete
       regular expression.

       Well now, the pattern really, _r_e_a_l_l_y wants to succeed, so
       it uses the standard regexp backoff-and-retry and lets \\\\DDDD****
       expand to just "AB" this time.  Now there's indeed
       something following "AB" that is not "123".  It's in fact
       "C123", which suffices.

       We can deal with this by using both an assertion and a
       negation.  We'll say that the first part in $$$$1111 must be
       followed by a digit, and in fact, it must also be followed
       by something that's not "123".  Remember that the
       lookaheads are zero-width expressions--they only look, but
       don't consume any of the string in their match.  So
       rewriting this way produces what you'd expect; that is,
       case 5 will fail, but case 6 succeeds:

           pppprrrriiiinnnntttt """"5555:::: ggggooootttt $$$$1111\\\\nnnn"""" iiiiffff $$$$xxxx ====~~~~ ////^^^^((((\\\\DDDD****))))((((????====\\\\dddd))))((((????!!!!111122223333))))//// ;;;;
           pppprrrriiiinnnntttt """"6666:::: ggggooootttt $$$$1111\\\\nnnn"""" iiiiffff $$$$yyyy ====~~~~ ////^^^^((((\\\\DDDD****))))((((????====\\\\dddd))))((((????!!!!111122223333))))//// ;;;;

           6666:::: ggggooootttt AAAABBBBCCCC

       In other words, the two zero-width assertions next to each
       other work like they're ANDed together, just as you'd use
       any builtin assertions:  ////^^^^$$$$//// matches only if you're at
       the beginning of the line AND the end of the line
       simultaneously.  The deeper underlying truth is that
       juxtaposition in regular expressions always means AND,
       except when you write an explicit OR using the vertical
       bar.  ////aaaabbbb//// means match "a" AND (then) match "b", although
       the attempted matches are made at different positions
       because "a" is not a zero-width assertion, but a one-width
       assertion.

       One warning: particularly complicated regular expressions
       can take exponential time to solve due to the immense



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       number of possible ways they can use backtracking to try
       match.  For example this will take a very long time to run

           ////((((((((aaaa{{{{0000,,,,5555}}}})))){{{{0000,,,,5555}}}})))){{{{0000,,,,5555}}}}////

       And if you used ****'s instead of limiting it to 0 through 5
       matches, then it would take literally forever--or until
       you ran out of stack space.

       VVVVeeeerrrrssssiiiioooonnnn 8888 RRRReeeegggguuuullllaaaarrrr EEEExxxxpppprrrreeeessssssssiiiioooonnnnssss

       In case you're not familiar with the "regular" Version 8
       regexp routines, here are the pattern-matching rules not
       described above.

       Any single character matches itself, unless it is a
       _m_e_t_a_c_h_a_r_a_c_t_e_r with a special meaning described here or
       above.  You can cause characters which normally function
       as metacharacters to be interpreted literally by prefixing
       them with a "\" (e.g. "\." matches a ".", not any
       character; "\\" matches a "\").  A series of characters
       matches that series of characters in the target string, so
       the pattern bbbblllluuuurrrrffffllll would match "blurfl" in the target
       string.

       You can specify a character class, by enclosing a list of
       characters in [[[[]]]], which will match any one of the
       characters in the list.  If the first character after the
       "[" is "^", the class matches any character not in the
       list.  Within a list, the "-" character is used to specify
       a range, so that aaaa----zzzz represents all the characters between
       "a" and "z", inclusive.

       Characters may be specified using a metacharacter syntax
       much like that used in C: "\n" matches a newline, "\t" a
       tab, "\r" a carriage return, "\f" a form feed, etc.  More
       generally, \_n_n_n, where _n_n_n is a string of octal digits,
       matches the character whose ASCII value is _n_n_n.
       Similarly, \x_n_n, where _n_n are hexidecimal digits, matches
       the character whose ASCII value is _n_n. The expression \c_x
       matches the ASCII character control-_x.  Finally, the "."
       metacharacter matches any character except "\n" (unless
       you use ////ssss).

       You can specify a series of alternatives for a pattern
       using "|" to separate them, so that ffffeeeeeeee||||ffffiiiieeee||||ffffooooeeee will match
       any of "fee", "fie", or "foe" in the target string (as
       would ffff((((eeee||||iiii||||oooo))))eeee).  Note that the first alternative
       includes everything from the last pattern delimiter ("(",
       "[", or the beginning of the pattern) up to the first "|",
       and the last alternative contains everything from the last
       "|" to the next pattern delimiter.  For this reason, it's
       common practice to include alternatives in parentheses, to
       minimize confusion about where they start and end.  Note



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       however that "|" is interpreted as a literal with square
       brackets, so if you write [[[[ffffeeeeeeee||||ffffiiiieeee||||ffffooooeeee]]]] you're really only
       matching [[[[ffffeeeeiiiioooo||||]]]].

       Within a pattern, you may designate subpatterns for later
       reference by enclosing them in parentheses, and you may
       refer back to the _nth subpattern later in the pattern
       using the metacharacter \_n.  Subpatterns are numbered
       based on the left to right order of their opening
       parenthesis.  Note that a backreference matches whatever
       actually matched the subpattern in the string being
       examined, not the rules for that subpattern.  Therefore,
       ((((0000||||0000xxxx))))\\\\dddd****\\\\ssss\\\\1111\\\\dddd**** will match "0x1234 0x4321",but not
       "0x1234 01234", since subpattern 1 actually matched "0x",
       even though the rule 0000||||0000xxxx could potentially match the
       leading 0 in the second number.

       WWWWAAAARRRRNNNNIIIINNNNGGGG oooonnnn \\\\1111 vvvvssss $$$$1111

       Some people get too used to writing things like

           $$$$ppppaaaatttttttteeeerrrrnnnn ====~~~~ ssss////((((\\\\WWWW))))////\\\\\\\\\\\\1111////gggg;;;;

       This is grandfathered for the RHS of a substitute to avoid
       shocking the sssseeeedddd addicts, but it's a dirty habit to get
       into.  That's because in PerlThink, the right-hand side of
       a ssss//////////// is a double-quoted string.  \\\\1111 in the usual double-
       quoted string means a control-A.  The customary Unix
       meaning of \\\\1111 is kludged in for ssss////////////.  However, if you get
       into the habit of doing that, you get yourself into
       trouble if you then add an ////eeee modifier.

           ssss////((((\\\\dddd++++))))//// \\\\1111 ++++ 1111 ////eeeegggg;;;;

       Or if you try to do

           ssss////((((\\\\dddd++++))))////\\\\1111000000000000////;;;;

       You can't disambiguate that by saying \\\\{{{{1111}}}}000000000000, whereas you
       can fix it with $$$${{{{1111}}}}000000000000.  Basically, the operation of
       interpolation should not be confused with the operation of
       matching a backreference.  Certainly they mean two
       different things on the _l_e_f_t side of the ssss////////////.














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